Optical encoders are used in a wide variety of contexts to determine movement and/or a position of an object with respect to some reference. Optical encoding is often used in mechanical systems as an inexpensive and reliable way to measure and track motion among moving components. For instance, printers, scanners, photocopiers, fax machines, plotters, and other imaging systems often use optical encoding to track the movement of an image media, such as paper, as an image is printed on the media or an image is scanned from the media.
One common technique for optical encoding uses an optical sensor and an optical encoder pattern (or encoding media). The optical sensor focuses on a surface of the optical encoder pattern. As the sensor moves with respect to the optical encoder pattern (or encoding media), or the encoder pattern moves with respect to the optical sensor, the optical sensor reads a pattern of light either transmitted through, or reflected by, the optical encoder pattern to detect the motion.
A typical optical encoder pattern is an alternating series of light and dark elements. As the encoder and sensor move relative to the one another, transitions from one element to the next in the pattern are optically detected. For instance, an encoder pattern could be an alternating pattern of holes, or optically transmissive windows, in an opaque material. In that case, an optical sensor can detect transitions from darkness to light passing through the holes or windows.
FIG. 1 illustrates a basic optical encoder 100 comprising an optical unit 103 including an optical emitter 101 and an optical sensor 102, and a light controlling member (optical encoder pattern) 105 disposed between the optical emitter 101 and the optical sensor 102. Optical emitter 101 is a light source comprising, for example, one or more light emitting diodes. In general, optical sensor 102 comprises one or more photo-detectors, for example, photodiodes or charge coupled devices (CCDs). Optical unit 103 and optical encoder pattern 105 can move relative to each other in a linear fashion longitudinally of optical encoder pattern 105.
In one common application, optical unit 103 is mounted on the printing head of a printer, optical encoder pattern 105 is fixed to a case of the printer, and optical unit 103 moves along the length of encoder pattern 105 when the printing head moves. As optical unit 103 moves along the length of optical encoder pattern 105, light from optical emitter 101 passing through (or reflecting from) optical encoder pattern 105 is sensed by one or more photo-detectors of optical sensor 102 to produce one or more signals that indicate the relative movement between optical unit 103 and optical encoder pattern 105. The output signal or signals from optical sensor 102 are then used by the printer to help control the movement of the printing head and/or paper in the printing process.
FIGS. 2A-B illustrate the relationship between optical encoder pattern 105 formed on a code strip 210, a photo-detector 220 of optical sensor 102, and an output signal produced by photo-detector 220 when optical encoder pattern 105 and the optical unit 103 (including photo-detector 220) move relative to each other.
As seen in FIG. 2A, optical encoder pattern 105 is an alternating pattern of rectangular shaped “light” elements 230 and “dark” elements 240, and photo-detector 220 also has a rectangular shape. In many cases, the light elements 230 comprise light-transmitting regions, which may be transparent regions or apertures in the code strip 210, so that light from the optical emitter 101 passes through light elements 230 of code strip 210 to optical sensor 102, but is blocked by dark elements 240 from reaching optical sensor 102. In another alternative arrangement, light elements 230 comprise light-reflecting regions which may be white or shiny, so that light from optical emitter 101 reflects back from light elements 230 of code strip 210 to optical sensor 102, but light is absorbed by dark elements 240 and not reflected to optical sensor 102. The discussion to follow is equally applicable to each of these configurations.
Photo-detector 220 produces an output signal that depends upon the amount of light it receives from optical pattern 105. As optical encoder pattern 105 and optical unit 103 (including photo-detector 220) move relative to each other, the amount of light received by photo-detector 220 varies from virtually no light when photo-detector 220 is aligned with a dark element 240 of optical encoder pattern 105, to a maximum amount of light when photo-detector 220 is aligned with a light element 230 of optical pattern 105. Assuming that optical encoder pattern 105 and optical unit 103 move relative to each other at a constant rate, then FIG. 2B shows the output signal of photo-detector 220.
As can be seen in FIG. 2B, the output signal of photo-detector 220 is a trapezoidal-shaped signal, with a flat top, a flat bottom, and a constant slope between the top and bottom. The arrangement of FIGS. 2A-B pertains specifically to a linear code strip 210. In some cases, a circular code wheel is used in place of the code strip 210, in which case either the light elements 230 and dark elements 240, or the photo-detector 220, has a trapezoidal shape instead of the rectangular shape. In either case, the photo-detector 220 produces the trapezoidal-shaped output signal shown in FIG. 2B.
However, there are some disadvantages to the arrangement illustrated in FIGS. 2A-B. In particular, from a feedback or control system standpoint, the trapezoidal-shaped output signal of the photo-detector is not very desirable. During the “flat spots” as the top and bottom, the photo-detector is not outputting any useful information regarding the relative movement between the optical sensor and the optical encoder pattern. Furthermore, the trapezoidal-shaped output signal is actually a series of ramp functions, and it is well known that a ramp function is not differentiable. So acceleration cannot be obtained. From a feedback or control system standpoint, it would be preferable if the output signal of the photo-detector was instead generally sinusoidal in nature.
What is needed, therefore, is an optical encoder whose photo-detector(s) produce a generally sinusoidal output signal in response to relative movement between the optical encoder pattern and the optical sensor.